I think it's because, comparitively, the size of the Lyre Valley and the size of the Borderlands are very much different, with the Valley being quite small in comparison. I don't think that such a small area, relatively speaking, would need super-detailed encounter charts; the new Bard's Gate book has them seperated by area around the valley, which have worked for me so far.

Maybe when they eventually get around to converting Stoneheart Valley to 5e we'll get some beefed up encounter charts, but I doubt it; I just don't think it needs it.

The Stoneheart Valley, specifically, doesn't have really beefy encounter tables because it's really meant to be a low level area. This is even explained away a bit in the book, in that, the Tradeway is a major transportation route that is regularly patrolled by the Waymarch.

If you're interested in areas to the north and east of Stoneheart Valley, north of Bard's Gate, I recommend picking up Sword of Air. It details the remainder of the Gulf of Akados region very well, with appropriate wandering monster tables. It also details Stoneheart Forest, which isn't covered in either of the other books, to my knowledge.